Top: DIB EW vs. A_V for the five average spectra. Blue and red points correspond to the two fitting procedures (see text). The dashed red curve represents the Zasowski et al. (2015) relation. The pink line is the Lan et al. (2015) EW vs. A_V relationship for the 6284 Å DIB, scaled to match the strength of the 15 273 Å DIB at A_V ≤ 1 (see text). Bottom: volume density of DIB carrier n(DIB) based on the simplified inversion of EWs (left scale). Also shown is the inverted volume density n(H) based on A_V data. The n(DIB) has been artificially scaled in such a way the two curves cross for the external layer. The ratio between the DIB carrier and H volume densities is also shown (right scale).